Bugdel: An Aspect-Oriented Debugging System

1. Bugdel: An Aspect-Oriented Debugging System Yoshiyuki Usui* (Tokyo Tech) Shigeru Chiba (Tokyo Tech) Asian workshop on AODS

2. What is debugging? • Purpose: locate bugs • Rough location: found by tracing • Display values of variables at many positions • Program execution is not stopped • More precise location: by breakpoints • Set at a few locations • Program execution is stopped Asian workshop on AODS

3. Beyond classical debugging • How to have more flexible tracing? • How to separate tracing code? • To minimize the risk of introducing bugs when adding / removing tracing code • How to debug with a JVM without debug mode (e.g. embedded JVMs)? • AOP can solve these problems Asian workshop on AODS

4. Debugging using AOP • Developers can specify many execution points with simple description • By pointcut • Trace all assignments to a field • set(int Point.x) (in AspectJ) • No need to enumerate all of target points • Separates tracing code • Developers add and remove the code without editing the source files • Reduce the risk of changing around tracing code by mistakes • Does not require JVM debug mode (e.g. JPDA) Asian workshop on AODS

5. Our contribution • Bugdel: AO system specialized for debugging • Solves problems found in general AOP systems when used for debugging • Relaxes constraints of modularization to allow useful debugging Asian workshop on AODS

6. Is classical AOP adapted? No! Asian workshop on AODS

7. Is classical AOP adapted? No! • Developers must learn many things only for debugging • The grammar of AOP language such as AspectJ • How to describe pointcuts and advices • The kinds of primitive pointcuts • Bugdel • Easy to use interface in the form of an Eclipse plug-in Asian workshop on AODS

8. GUI of Bugdel Bugdel editor is used to specify pointcuts Bugdel Editor Bugdel view shows information of advices Bugdel View Asian workshop on AODS

9. GUI based programming 1/2 • Candidate pointcuts are shown • Pointcuts are related to the selected class, method or field on the source file Proposes “get” and “set” pointcut to the field x in the class Point Asian workshop on AODS

10. GUI based programming 2/2 • Advice Dialog • Select • Pointcut designators • Before or after advice • Input • Name of the class, method and field • Advice body • Wild cards “*” are accepted in names • Developers need not learn the kinds of primitive pointcuts Asian workshop on AODS

11. Is classical AOP adapted? No! • When debugging, developers consider source files • They want to execute tracing code at a certain line in a source file • These concepts are not available in classical AOPsystem’s pointcuts • Bugdel provides Line and AllLines pointcuts to pointcut lines in source files • Necessary for a debugging system Asian workshop on AODS

12. Line, AllLines pointcuts • Line pointcut • Selects the beginning of a line as a join point • Easy selection with the mouse • Developer can trace at that certain line • Useful for debugging but breaks class modularization • AllLines pointcut • Selects all the lines in a method Asian workshop on AODS

13. Is classical AOP adapted? No! • When debugging, developers often trace the local variables • Not possible to access local variables in advices • Advice language are too limited Asian workshop on AODS

14. Bugdel’s advice language 1/2(Access to the local variables) • In Bugdel, the local variables around a join point are accessible from an advice body • Developers can trace local variables by using advices • Useful for debugging but breaks class modularization Advice class Server{ void run(){ int counter = 10; service(); … } Pointcut : call of service() method Advice body: System.out.println(counter); This is possible only in Bugdel, but not in AspectJ. Asian workshop on AODS

15. Bugdel’s advice language 2/2 • Provides many powerful mechanisms • Reflection • thisJoinPoint.variables • thisJoinPoint.line • thisJoinPoint.filePath • Etc. • Built-in methods • bugdel.Bugdel.openEditor(..) • bugdel.Bugdel.jump(int line) • These mechanisms are specific and useful for debugging Asian workshop on AODS

16. Breakpoint emulation • Bugdel makes it easy to emulate breakpoints • Useful when the JVM has no debug mode • e.g. embedded JVMs • Let’s watch the demo and see the implementation details later Asian workshop on AODS

17. Breakpoint emulation: demo Asian workshop on AODS

18. Demo pointcut Step execution using AllLines pointcut • Pointcuts specify breakpoint location, e.g. • Method entry: MethodExecution • Field watch: FieldSet, FieldGet • Line • Emulate step execution • AllLines pointcut • Selects beginnings of the all lines in a method as a join point • Suspend the thread at all lines Asian workshop on AODS

19. Demo advice System.err.println(thisJoinPoint.location);Object[][] vs = thisJoinPoint.variables;bugdel.Bugdel.printVariables(vs);String file = thisJoinPoint.filePath;int line = thisJoinPoint.line;bugdel.Bugdel.openEditor(“localhost”,5555,file,line);javax.swing.JOptionPane.showMessageDialog(null,“OK?”); Asian workshop on AODS

20. Blocks the thread until OK button is pressed Demo advice (1/3) System.err.println(thisJoinPoint.location);Object[][] vs = thisJoinPoint.variables;bugdel.Bugdel.printVariables(vs);String file = thisJoinPoint.filePath;int line = thisJoinPoint.line;bugdel.Bugdel.openEditor("localhost",5555,file,line);javax.swing.JOptionPane.showMessageDialog(null,"OK?"); Asian workshop on AODS

21. Eclipse＆Bugdel 2. Send “filePath” and “line” 1.The program calls openEditor(..) 3.Open the file and highlight the line JVM Demo advice (2/3) • Highlight the breaking location on the source files • File names and line numbers are different for every joint point • bugdel.Bugdel.openEditor(…) • Opens the source file with Bugdel editor and highlights the line System.err.println(thisJoinPoint.location);Object[][] vs = thisJoinPoint.variables;bugdel.Bugdel.printVariables(vs);String file = thisJoinPoint.filePath;int line = thisJoinPoint.line;bugdel.Bugdel.openEditor("localhost",5555,file,line);javax.swing.JOptionPane.showMessageDialog(null,"OK?"); Asian workshop on AODS

22. Demo advice (3/3) System.err.println(thisJoinPoint.location);Object[][] vs = thisJoinPoint.variables;bugdel.Bugdel.printVariables(vs);String file = thisJoinPoint.filePath;int line = thisJoinPoint.line;bugdel.Bugdel.openEditor("localhost",5555,file,line);javax.swing.JOptionPane.showMessageDialog(null,"OK?"); • Print out all variables at the breakpoint Asian workshop on AODS

23. Print all variables at the breakpoint • thisJoinPoint.variables • Type is Object[][] • It is a list of the local variables and the fields visible at the join point • It includes name, value, type and modifier of each variable Implementation of printVariables(…): Object[][] v = thisJoinPoint.variables;for(int i=0; i<v.length; i++){ System.err.print(v[i][0]+”=“+v[i][1]+”,”);} Asian workshop on AODS

24. Bugdel’s weaver • Bugdel’s weaver transforms the bytecode • Uses Javassist, our bytecode transformation system • Aspects are woven into the class files • Eclipse or Bugdel are not necessary to run the debugged program • Possible to debug in any JVM without debug mode Asian workshop on AODS

25. Related Work 1 • General purpose AO systems • AspectJ, AspectWerks, JBoss AOP • Class modularization is important • Bugdel • Domain specific (debugging) AO system • Line pointcut • Access to local variables around the join point • These features break class modularization • They depend on the method implementation • If they are provided, developers can not change the method implementation easily • However, usefulness for debugging is more important than class modularization Asian workshop on AODS

26. Related Work 2 • Traditional debuggers • Essentially provides breakpoints mechanism • Program execution is stopped • May facilitate tracing • JVMs must have a debug mode • Bugdel • Same user interface and functions • Useful for tracing • Line and AllLines pointcuts for breakpoints • The JVM needs not have a debug mode • Even breakpoints can be emulated • However, developers must set breakpoints at weaving time Asian workshop on AODS

27. Conclusion • Bugdel • Eclipse plug-in • GUI based programming • AO system specialized for debugging • Line pointcut, AllLines pointcut • Access to the local variables around the join point • thisJoinPoint.varables • bugdel.Bugdel.openEditor(..) • Etc. • Useful for tracing • Breakpoint emulation • On any JVM without debug mode Asian workshop on AODS

28. Please give Bugdel a try! • Free software (CPL license) • Compatible with Eclipse 3.1.x, 3.0.x • Downloadable from: http://www.csg.is.titech.ac.jp/~usui/bugdel/ • Or look for “Bugdel” in Google ;-) Asian workshop on AODS